Fluid Dynamic Bearing Device

ABSTRACT

It is an object of the present invention to provide a bearing member having a highly accurate dynamic pressure generating portion stably at low cost. Dynamic pressure grooves ( 8   a   1 ) and ( 8   a   2 ) are formed as the dynamic pressure generating portions on an inner circumferential surface of the electroforming portion ( 10 ) by an electroforming process, and injection molding is performed by using a resin while inserting the electroforming portion ( 10 ), thereby forming a bearing member ( 8 ). A shaft member ( 2 ) is inserted into an inner circumference of the bearing member.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a bearing device for rotatablysupporting a shaft member by a dynamic pressure action of fluid, whichis generated in a bearing gap. This type of the bearing device is calleda “fluid dynamic bearing device”, and is suitable for use in a spindlemotor mounted in an information apparatus, for example, a magnetic diskdevice such as an HDD and an FDD, an optical disk device such as aCD-ROM/RAM, or a magneto-optical disk device such as an MD/MO, in apolygon scanner motor mounted in a laser beam printer (LBP) or the like,or in a small motor mounted in an electrical apparatus such as an axialfan.

2. Description of the Related Art

In the fluid dynamic bearing device, usually, a shaft member issupported in a non-contact fashion with a bearing member in a radialdirection by a dynamic pressure action of fluid, which is generated in aradial bearing gap. In order to generate the dynamic pressure action ofthe fluid in the radial bearing gap, dynamic pressure grooves arranged,for example, in a herringbone configuration are formed as a dynamicpressure generating portion in a predetermined region of at least anyone of an outer circumferential surface of the shaft member and an innercircumferential surface of the bearing member, which is oppositethereto.

Meanwhile, for the above-mentioned fluid dynamic bearing device for theinformation apparatus, extremely high rotational accuracy is required,and hence, it is necessary to form the dynamic pressure grooves withextremely high accuracy. As a method of forming this type of the dynamicpressure grooves on the inner circumferential surface of the bearingmember, a form rolling is known (for example, see JP 10-196640 A).

In the invention described in JP 10-196640 A described above, a rollingmachine having a plurality of protrusions is inserted into the innercircumference of the bearing member, and the rolling machine is thenallowed to travel thereinto in an axial direction while being rotatedalternately in a clockwise direction and a counter clockwise direction,thereby forming the dynamic pressure grooves in the inner circumferenceof the bearing member. In the method of forming the dynamic pressuregrooves by the form rolling as described above, variation is prone tooccur in a shape of the grooves owing to characteristics of the method,and it is difficult to mass-produce highly accurate grooves stably atlow cost.

Further, for the above-mentioned bearing device for use in theinformation apparatus, much higher rotational accuracy is required.Therefore, it becomes important to set, with high accuracy, a gap(clearance) between the inner circumferential surface of the bearingmember and the outer circumferential surface of the shaft member, whichis opposite thereto.

SUMMARY OF THE INVENTION

In this connection, it is an object of the present invention to providea fluid dynamic bearing device having high rotational accuracy.

To achieve the above-mentioned object, a fluid dynamic bearing deviceaccording to the present invention includes: a bearing member; and ashaft member inserted into an inner circumference of the bearing member,and is characterized in that the bearing member is formed by injectionmolding in which an electroforming portion is inserted, and any one ofan inner circumferential surface of the electroforming portion and anouter circumferential surface of the shaft member, which is opposed tothe inner circumferential surface of the electroforming portion isprovided with a dynamic pressure generating portion formed thereon.

The electroforming portion is formed by an electroforming process offorming a metal layer by causing metal ions to deposit on a mastersurface. Such the formation of the metal layer can also be performed byusing a method similar to electroless plating, as well as a methodsimilar to electrolytic plating. In a case of forming the dynamicpressure generating portion on the electroforming portion, a formingportion is formed into a non-perfect circular shape in cross section,which corresponds to a shape of the dynamic pressure generating portion,and in a case of forming the dynamic pressure generating portion on theshaft portion, the forming portion is formed into a perfect circularshape in cross section. Owing to characteristics of the electroformingprocess, the forming portion of a master shaft is transferred to theelectroforming portion with high accuracy, and a surface to which theforming portion is transferred has a surface accuracy following to asurface accuracy of the master shaft. Accordingly, if the surfaceaccuracy of the master shaft (in particular, of the forming portionthereof) is enhanced in advance, the inner circumferential surface ofthe bearing member can be molded with high accuracy, thus making itpossible to set the bearing gap with high accuracy.

For example, the accuracy of the forming portion of the master shaft isenhanced in advance by forming the dynamic pressure generating portionon the inner circumferential surface of the electroforming portion, thusmaking it possible to form the dynamic pressure generating portion withhigh accuracy. The bearing member may be fabricated through the stepsof: fabricating a master shaft having a forming portion corresponding toa shape of a dynamic pressure generating portion on an outercircumference of the master shaft (master shaft fabrication step);forming an electroforming portion on the outer circumference of themaster shaft including the forming portion (electroforming step);performing injection molding while inserting the electroforming portionafter the electroforming portion is formed (molding step); andseparating the master shaft and the electroforming portion from eachother after the injection molding (separation step).

In the separation step after the above-mentioned injection molding, themaster shaft and the electroforming portion are separated from eachother. Such the separation can be performed, for example, by allowing aninner stress in a radial expansion direction, which is accumulated inthe electroforming portion as the electroforming process is performed,to be released to make the inner circumference of the electroformingportion be thus expanded in diameter. When a diameter expansion amountof the electroforming portion is insufficient only by an operationdescribed above, the master shaft and the electroforming portion areheated or cooled to give a difference in thermal expansion amounttherebetween. Then, it becomes possible to smoothly draw out the mastershaft from the inner circumference of the bearing member withoutdamaging the dynamic pressure generating portion formed on theelectroforming portion.

As the shaft member of the bearing device, the master shaft used at thetime of molding the electroforming portion may be directly used, and aseparate member from the master shaft may be used as well. In the lattercase, the separated master shaft can be repeatedly used in theelectroforming process. Accordingly, a highly accurate bearing membercan be mass-produced stably.

As an example of the dynamic pressure generating portion, a plurality ofdynamic pressure grooves arranged in a herringbone configuration and thelike can be exemplified. A dynamic pressure groove pattern including thedynamic pressure grooves forms an extremely complicated shape. Even inthis case, if a forming portion having a portion corresponding to thedynamic pressure groove pattern is formed in advance on the outercircumference of the master shaft in the master shaft fabrication step,the shape of the forming portion is precisely transferred by theelectroforming process. Accordingly, the highly accurate dynamicpressure groove pattern can be formed easily at low cost. Note that thedynamic pressure generating portion formed on the electroforming portionmay be formed, for example, of a plurality of circular arc surfaces aswell as the above-mentioned dynamic pressure grooves.

In the molding step, the injection molding is performed while insertingthe master shaft which has undergone the electroforming process into amold (insert molding), thereby integrally molding the bearing membercomposed of the mold portion and the electroforming portion. In theinsert molding, a highly accurate part is integrally molded only byenhancing accuracy of the mold and positioning the electroformingportion with high accuracy. Accordingly, if the electroforming portionand the master shaft are thereafter separated from each other, aresultant molded article can be directly used as the bearing member forthe bearing device. Owing to the characteristics of the electroformingprocess, the outer circumferential surface of the electroforming portionis formed to have a rough surface. Accordingly, when the insert moldingis performed as described above, the injection molding material entersthe outer circumferential surface of the electroforming portion, so anadhesion force there between becomes stronger due to an anchor effect.

It is preferable to form a flange on the electroforming portion prior tothe molding step. By forming the flange, the flange and the mold portionare prevented from being detached from each other or being rotated withrespect to each other after the injection molding. Accordingly, muchhigher adhesion force can be obtained between the electroforming portionand the mold portion. In particular, by forming the outercircumferential surface of the flange into the non-perfect circularshape, it becomes possible to obtain a higher effect to prevent therotation as described above. The flange also includes the one whichextends in an oblique direction from the axis center (refer to FIG. 7),as well as the one which extends in a perpendicular direction to theaxis center (refer to FIG. 6).

The flange of the electroforming portion can be formed by plasticallydeforming the electroforming portion. For example, if an end surface ofthe electroforming portion, which is brought into intimate contact withthe outer circumference of the master shaft, is pressurized in the axialdirection, the end portion of the electroforming portion is plasticallydeformed to the radial outer side thereof since a portion to bepressurized cannot be deformed to the inner diameter side thereof whichis brought into intimate contact with the master shaft. Thus, it becomespossible to easily form the outward flange. In particular, in the caseof molding the electroforming portion by the injection molding, if theelectroforming portion is partially deformed plastically by clamping ofthe metal mold, after the flange is formed, the resin or the metal isinjected into a cavity while leaving the electroforming portion in whichthe flange is formed as it is, thus making it possible to form thebearing by the insert molding. Thus, it becomes possible to fabricatethe bearing at low cost without requiring a special process for formingthe flange. Note that, since the outer circumferential surface of theplastically deformed flange usually becomes a non-perfect circularshape, it becomes possible to form the flange having the outercircumferential surface with the non-perfect circular shape withoutparticularly adding another step.

If the flange formed by the plastic deformation is formed on one endportion of the electroforming portion or both end portions thereof, aninfluence of the plastic deformation becomes less for the axial centerportion of the bearing surface. Hence, a bearing surface accuracy on theaxial center portion of the bearing surface, which is important in termsof a function of the bearing, can be prevented from being decreased.

By the way, if a pressurizing force applied to the electroformingportion is too large in the case of forming the flange by the plasticdeformation, there is an apprehension that, owing to an impact at thattime, the inner circumferential surface of the electroforming portion,which is brought into intimate contact with the master shaft, may bepeeled off from the outer circumferential surface of the master shaft.In order to prevent such the situation, it is desirable to set a changeof an axial length of the electroforming portion between before andafter the plastic deformation within 50% of an axial length of theelectroforming portion after the plastic deformation. To be specific,when an axial length of the electroforming portion before the plasticdeformation of the flange is L2, and the axial length of theelectroforming portion after the plastic deformation is L1, it isdesirable to set L1 and L2 so as to satisfy the following expression:

0<A/L1≦0.5  (where A=L2−L1)

As a material subjected to the injection molding in the molding step, ametal material, ceramics, and the like are usable as well as the resinmaterial. In the case of using the metal material, it is possible touse, for example, injection molding of low melting point metal such as amagnesium alloy, so-called MIM molding for performing the injectionmolding for metal powder and a binder with each other, followed bydegreasing and sintering, and the like. In the case of using theceramics, it is possible to use, for example, so-called CIM molding forperforming the injection molding for ceramic powder and the binder witheach other, followed by degreasing and sintering. In general, in thecase of using the resin material, features are obtained that an obtainedarticle is excellent in moldability and lightweight. In the case ofusing the metal material, features are obtained that an obtained articleis excellent in rigidity, conductivity, and heat resistance. Further, inthe case of using the ceramics, features are obtained that an obtainedarticle is more lightweight than the metal material and excellent inrigidity, heat resistance, and so on.

By the way, when the electroforming portion is formed into a cylindricalshape, it is conceived that a residual stress in a diameter expansiondirection is applied to an inner composition of the electroformingportion after being formed. Meanwhile, in the case of using the resin asthe injection molding material, the cylindrical resin mold portion isgoing to be shrunk as being solidified. Hence, after the resin moldingof the electroforming portion, the outer circumferential surface of theelectroforming portion and the inner circumferential surface of theresin mold portion are pressed against each other. In addition, whilethe inner circumferential surface of the electroforming portion becomesa smooth surface corresponding to the outer circumferential surface ofthe master shaft, the outer circumferential surface of theelectroforming portion becomes a rough surface in general. Accordingly,the resin enters irregularities of the surface of the electroformingportion after the resin molding, and an anchor effect is generated.Owing to composite action of the above, the strong adhesion forcebetween the electroforming portion and the resin mold portion can beobtained.

In order to put the electroforming bearing as described above intopractical use as an industrial product, it becomes necessary tostably-ensure much stronger-adhesion force between the electroformingportion and the mold portion. Meanwhile, even if the strong adhesionforce is obtained, if other bearing characteristics such as the bearingsurface accuracy are traded off therefor, the electroforming bearing isinhibited from being put into practical use.

In this connection, in the present invention, the resin is used as theinjection molding material of the bearing member, and a moldingshrinkage of the resin is set within a range of 0.02% to 2.0% inclusive.By setting the molding shrinkage of the resin at 0.02% or more, ashrinkage force caused in the resin mold portion at the time when themolten resin is solidified increases. Accordingly, a required adhesionforce can be surely ensured between the electroforming portion and themold portion. Meanwhile, when the molding shrinkage of the resin is toolarge, the shrinkage force of the mold portion becomes excessive,resulting in an apprehension that the electroforming portion may bedeformed owing to propagation of the shrinkage force. However, an upperlimit of the molding shrinkage is set at 2.0%, thus making it possibleto avoid this type of harmful effect.

According to the present invention, it is desirable that the dynamicpressure generating portion has a cross section in a radius direction,in which a radius r1 of a virtual circle inscribed to the innercircumferential surface of the electroforming portion is larger than aradius r2 of a virtual circle circumscribed to the outer circumferentialsurface of the shaft member, and in which a sum of a circularity of thebearing surface of the electroforming portion and a circularity of theouter circumferential surface of the shaft member is 4 μm or less.

As described above, the radius r1 of the inscribed circle of the bearingsurface of the electroforming portion and the radius r2 of thecircumscribed surface of the shaft member are set to r1>r2, and theminimum clearance between the bearing surface of the bearing member andthe outer circumferential surface of the shaft member can be thusensured, thereby making it possible to avoid contact and sliding betweenthe shaft member and the bearing member when they rotate relatively toeach other as much as possible to obtain a stable rotation supportingstate. Meanwhile, even when the above-mentioned minimum clearance isensured, if circularities of the bearing surface of the bearing memberand the outer circumferential surface of the shaft member are too high,the bearing gap is uneven in the circumferential direction. Accordingly,rotational accuracy including accuracy such as radial run out of theshaft decreases, and there is also a fear of a lifetime of the bearingdecreasing owing to abrasion of both of the surfaces on such the contactand sliding portion. According to verification by the inventor of thepresent invention from such a viewpoint, it has been found out that theabove-mentioned harmful effect can be avoided when the sum of thecircularity of the bearing-surface of the electroforming portion and thecircularity of the outer circumferential surface of the shaft member is4 μm or less.

In this case, a master used at the time of the electroforming processcan be directly used as the shaft member. The surface shape of themaster is transferred precisely in a micron order to the innercircumferential surface of the electroforming portion, which serves asthe bearing surface. Accordingly, when the sum of the circularity of thebearing surface of the electroforming portion and the circularity of theouter circumferential surface of the shaft member is defined asdescribed above, the circularity of the outer circumferential surface ofthe shaft member as the master becomes approximately a half (2 μm) ofthe above-mentioned sum. Hence, if the master surface is subjected inadvance to a finishing process so that the circularity is theabove-mentioned numeric value or less, it becomes possible to stablyobtain good rotational accuracy and a long lifetime of the bearing.

As the shaft member, a member fabricated separately from the master mayalso be used as well as the master. Also in this case, if a surface ofthe member is finished so that the circularity thereof is theabove-mentioned numeric value or less, which is half of theabove-mentioned sum, good rotational accuracy can be obtained stably.

Note that, the radii r1 and r2 and the circularities are measured oncommon cross sections in the radius direction. It is necessary that thecross sections in the radius direction be taken at a few arbitrary spots(desirably, three or more spots at an equal interval) in the axialdirection, and that the above-mentioned conditions be satisfied at therespective positions thus extracted. Note that the “circularity”mentioned here means a difference between radii of two concentricgeometric circles in the case where an interval of the two concentriccircles becomes the minimum when a circular body is sandwiched by thetwo concentric geometric circles (refer to FIG. 36).

On the electroforming portion, a thrust bearing surface for supportingthe end portion of the shaft member in the thrust direction can beformed. In this case, it is not necessary to incorporate a member(thrust plate and the like) constructing the thrust bearing surface intothe main body by means such as press-fitting and adhesion. Accordingly,the number of steps and the number of parts are reduced, thus making itpossible to achieve cost reduction of the bearing device.

As the thrust bearing portion, a dynamic bearing for supporting theshaft member in a non-contact fashion in the thrust direction by adynamic pressure action of fluid, which is generated in a thrust bearinggap between the thrust bearing surface and an end surface of the shaftmember, the end surface being opposite thereto, can also be used as wellas a pivot bearing for supporting the shaft member in a contact-fashionin the thrust direction on the thrust bearing surface. The dynamicbearing can be constructed, for example, by forming a plurality ofdynamic pressure grooves on any one of the thrust bearing surface andthe end surface of the shaft member, which is opposite thereto.

In the case where the thrust bearing portion is formed of the dynamicbearing, and where the dynamic pressure grooves are formed on the thrustbearing surface, if a thrust bearing surface forming portion having anirregular shape corresponding to a shape of the dynamic pressure groovesis formed in advance on the shaft end of the master shaft, the dynamicpressure grooves of the thrust bearing surface can be formed by theelectroforming process with high accuracy. On the other hand, in thecase where the dynamic pressure grooves are formed on the end surface ofthe shaft member, the electroforming process is performed by a mastershaft in which the shaft end is formed into a flat surface, and thethrust bearing surface is formed into a flat surface shape which doesnot have the dynamic pressure generating portion. Then, after theelectroforming portion and the master shaft are separated from eachother, a shaft member in which the dynamic pressure grooves are formedin advance on the end surface in another step is inserted into the innercircumference of the shaft member, and the dynamic bearing is thusconstructed.

In the case of using the master shaft as the shaft member, anotherthrust bearing surface may also be formed on the other end of the mastershaft, as well as the forming portion for forming the thrust bearingsurface is formed on one end of the master shaft. In this case, if thethrust bearing surface is formed by the forming portion, and after themaster shaft and the electroforming portion are separated from eachother, the master shaft is reversed and inserted into the innercircumference of the bearing member, the thrust bearing portion can beconstructed between the bearing constructing portion of the master shaftand the thrust bearing surface of the electroforming portion. For thebearing constructing portion of the master shaft, for example, there isconsidered a construction in which the plurality of dynamic pressuregrooves are formed on the end surface, a construction in which the endsurface is formed into the flat surface, or a construction in which theshaft end is formed into a spherical shape. In the case of the formertwo constructions, the dynamic bearing is composed of the bearingconstructing portion of the master shaft and the thrust bearing surface,and in the case of the latter one construction, the pivot bearing iscomposed of the bearing constructing portion and the thrust bearingsurface.

The bearing device having the above-mentioned construction can bepreferably used for a motor, for example, a spindle motor for the diskdevice such as the HDD. This motor is characterized by beinginexpensive, and in addition, in that rotational accuracy and durabilityare high.

The above description shows the case where the dynamic pressuregenerating portion is formed on any one of the inner circumferentialsurface of the electroforming portion and the outer circumferentialsurface of the shaft member, which is opposite thereto. However, therespective constructions described above can also be applied similarlyto the case (perfect circular bearing) where any of the innercircumferential surface of the electroforming portion and the outercircumferential surface of the shaft member, which is opposite thereto,has a perfect circular shape in cross section, which does not have thedynamic pressure generating portion. In this case, the master shaft isformed into the perfect circular shape in cross section. Thus, theradial bearing surface (inner circumferential surface of theelectroforming portion) of the bearing member is formed into the perfectcircular shape. Accordingly, the shaft member with the perfect circularshape is inserted into the inner circumference of the bearing memberafter the master shaft and the electroforming portion are separated fromeach other, and a radial bearing gap with the perfect circular shape isthus formed between the radial bearing surface with the perfect circularshape and the outer circumferential surface of the shaft member with theperfect circular shape in cross section, which is opposite thereto.

According to the present invention, the following effects can beobtained.

(1) It is possible to set the bearing gap at high accuracy, and thus thebearing performance of the bearing device can be enhanced.(2) A bearing member having a highly accurate dynamic pressuregenerating portion can be provided stably at low cost. In addition,powder that may be generated by cutting is not generated when thedynamic pressure grooves are formed, and an occurrence of contaminationcan be avoided.(3) Through the flange portion, the electroforming portion and the moldportion are prevented from being detached from each other or rotatedwith respect to each other, and accordingly, the adhesion force betweenthe electroforming portion and the mold portion can be further enhanced.(4) The bearing device can exert bearing performance which is stable fora long period of time, and the number of parts and the number ofassembly man-hours can be reduced, thus making it possible to achievecost reduction.(5) Strong adhesion force can be stably obtained between theelectroforming portion and the mold portion. Meanwhile, deformation ofthe electroforming portion owing to the shrinkage of the mold can berestricted, thus making it possible to obtain higher bearingperformance.(6) Rigidity, heat resistance, conductivity, and the like of the bearingcan be enhanced. Accordingly, the electroforming bearing becomes usableeven under a severe environment with a high load, a high temperature,and the like, thus making it possible to contribute to expansion of thepurpose of the electroforming bearing.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A description will be made of embodiments of the present invention basedon the drawings.

A bearing member 8 having a construction of the present invention, whichis shown in FIG. 1, is fabricated through the steps of: fabricating amaster shaft (refer to FIG. 2A); masking a spot of the master shaft 12,which requires the masking (refer to FIG. 2B); forming an electroformingshaft 11 by performing an electroforming process for an unmasked portionof the master shaft 12 (refer to FIG. 2C); forming the bearing member 8by molding an electroforming portion 10 of the electroforming shaft 11with a resin and the like (refer to FIG. 5); and separating theelectroforming portion 10 and the master shaft 12 from each other.

The master shaft 12 shown in FIG. 2A is formed of a conductive metalmaterial, for example, stainless steel subjected to a quenchingtreatment. As a matter of course, as long as moldability of theelectroforming portion 10 is good, metal materials other than thestainless steel, for example, a nickel alloy, a chromium alloy, and thelike can also be used. Even a nonmetal material such as ceramics can beused as the master shaft by being subjected to a conducting treatment(for example, by forming a conductive metal coating film on a surfacethereof). It is desirable that a surface of the master shaft 12 besubjected in advance to a surface treatment for reducing a frictionalforce between the electroforming portion 10 and itself, for example, tofluorine resin coating. In addition to a solid shaft, the master shaft12 can also be formed of a hollow shaft or a solid shaft formed byfilling a hollow portion with a resin.

As show in FIG. 2A, on a region of an outer circumferential surface 12 aof the master shaft 12, which becomes a portion where the electroformingportion 10 is to be formed, a forming portion N having an irregularshape corresponding to a shape of radial bearing surfaces A describedlater is formed. Aspects of irregularities of the forming portion N andthe radial bearing surfaces A are completely opposite to each other, andprotruding portions of the radial bearing surfaces correspond torecessed portions 12 a 1 and 12 a 2, of the forming portion N. Theexample shown in the figure illustrates the case where the recessedportions 12 a 1 and 12 a 2 are formed into a configuration correspondingto a dynamic pressure groove pattern of a herringbone shape; however,the recessed portions 12 a 1 and 12 a 2 can also be formed into aconfiguration corresponding to a dynamic pressure groove pattern of aspiral shape.

The forming portion N is formed, for example, by using a surfacetreatment process such as etching as well as a cutting process and amachining process such as a pressing process. Accuracy of the outercircumferential surface 12 a of the master shaft 12 including theforming portion N directly affects molding accuracy of a dynamicpressure generating portion, and eventually, bearing performance of adynamic bearing. Accordingly, it is necessary to increase in advanceaccuracies in circularity, cylindricality, surface roughness, and thelike, which are important for functions of the bearing device.

In the masking step shown in FIG. 2B, a masking 13 is applied to theouter circumferential surface 12 a of the master shaft 12, except forthe forming portion N. It is possible to use the existing articles whichare non-conductive and have corrosion resistance to an electrolyticsolution as a material of the masking 13.

The electroforming process shown in FIG. 2C is performed in such amanner that the master shaft 12 subjected to the masking process isimmersed into an electrolytic solution containing metal ions of Ni, Cu,or the like to cause the target metal to deposit onto the surface of themaster shaft 12 by applying power to the electrolytic solution. In theelectrolytic solution, a sliding material such as carbon or a stressrelaxing material such as saccharin may also be contained according toneeds. A type of such the electro deposited metal is selected asappropriate according to physical properties and chemical properties,such as hardness and fatigue strength, which are required for thedynamic pressure generating portion. With regard to a thickness of theelectroforming portion 10, if the electroforming portion 10 is toothick, releasability thereof from the master shaft 12 decreases, and ifthe electroforming portion 10 is too thin, durability thereof decreases,or the like. Accordingly, the thickness is optimally set according torequired bearing performance, a size of the bearing, and further, to apurpose thereof and the like. For example, in a rotational bearing witha shaft diameter of 1 to 6 mm, it is preferable to set the thicknesswithin a range of 10 to 200 μm.

As described above, the electroforming portion 10 is formed by using themethod similar to electrolytic plating. Further, the electroformingportion 10 may also be formed by using a method similar to electrolessplating.

Through the steps described above, as shown in FIG. 2C, theelectroforming shaft 11 is formed, in which the cylindricalelectroforming portion 10 adheres to the region (forming portion N) ofthe outer circumferential surface 12 a of the master shaft 12, whichexcludes the masking 13. At this time, onto an inner circumferentialsurface of the electroforming portion 10, the irregular shape of theforming portion N formed on the outer circumferential surface 12 a ofthe master shaft 12 is transferred, thus forming the plurality ofdynamic pressure grooves as the dynamic pressure generating portion.Note that, when a coating material 3 for the masking is thin, in somecases, as shown by the broken lines of FIG. 3, both ends of theelectroforming portion 4 protrude to the coating material 3 side, andtapered chamfered portions 4 a are formed on the inner circumferentialsurface of the electroforming portion.

Next, the electroforming shaft 11 is conveyed, for example, to a moldingstep shown in FIG. 4, and injection molding (insert molding) using aresin material is performed by using the electroforming shaft 11 as aninsert part.

In this molding step, the electroforming shaft 11 is supplied to a metalmold composed of an upper mold 15 and a lower mold 16 in a state wherean axial direction thereof is in parallel with a clamping direction(vertical direction in FIG. 4) of the metal mold. In the lower mold 16,a positioning hole 18 corresponding to an outer diameter dimension ofthe master shaft 12 is formed. Into the positioning hole 18, a lower endof the electroforming shaft 11 conveyed from the previous step isinserted, and positioning is effected for the electroforming shaft 11.In the positioning state, a lower end surface of the electroformingportion 10 in the electroforming shaft 11 engages with a molding surfaceof the lower mold 16, and an upper end of the electroforming portion 10protrudes from a parting line P.L. of the metal mold toward theother-half mold (upper mold 15 in this embodiment). A depth L3 of thepositioning hole 18 is larger than a distance L4 between a lower end ofthe master shaft 12 and a lower end of the electroforming portion 10(L3>L4). Hence, in a state before swaging, a lower end surface of themaster shaft 12 is floated from a bottom of the positioning hole 18. Byadjusting an amount of the floating, an amount of plastic deformation ofa flange formed on the lower end of the electroforming portion 10 can bechanged.

In the above-mentioned upper mold 15, a guide hole 19 is formedcoaxially with the positioning hole 18. A depth L5 of the guide hole 19is sufficient if the depth L5 is to an extent where an upper end of themaster shaft 12 does not reach or contact a bottom of the guide hole 19at a time of swaging shown in FIG. 5 (note that the lower end of themaster shaft 12 reaches and contacts the bottom of the positioning hole18).

In the metal mold described above, when the movable mold (upper mold 15in this embodiment) is made to approach the fixed mold (lower mold 16 inthis embodiment) to perform swaging, first, the upper end of the mastershaft 12 is inserted into the guide hole 19, and the master shaft 12 issubjected to centering, and further, an upper end surface of theelectroforming portion 10 abuts on a molding surface of the upper mold15. The electroforming shaft 11 is entirely pushed downward by furtherapproach of the upper mold 15. Then, as shown in FIG. 5, the lower endportion of the electroforming portion 10, which abuts on the moldingsurface of the lower mold 16, and the upper end portion of theelectroforming portion 10, which abuts on the molding surface of theupper mold 15, are individually deformed plastically to an outerdiameter side, and flanges 20 are formed on both axial ends of theelectroforming portion 10. By changing a structure of the metal mold, itis also possible to form the flange 20 on only one axial end of theelectroforming portion 10.

After the clamping is completed, the resin material is injected into acavity 17 through a spool 21, a runner 22, and a gate 23, and the insertmolding is performed. As the resin material, the one is preferable,which is excellent in mechanical strength as well as oil resistance,heat resistance, and the like. It is possible to use, for example, highperformance crystalline polymer such as liquid crystal polymer (LCP), apolyphenylene sulfide (PPS) resin, a polyacetal (POM) resin, and apolyamide (PA) resin as the resin material. As a matter of course, theseare mere examples, and it is possible to select resin materialsappropriate for the purpose and using environment of the bearing from avariety of the existing resin materials. According to needs, a varietyof fillers such as a reinforcement (in any form including fiber andpowder) and a lubricant may also be added to the resin material.

As the resin material, the one is selected, in which a molding shrinkage(predicted value after the addition of the filler) is within a range of0.02% to 2.0% inclusive (preferably, a range of 0.05% to 1.0%inclusive). According to the verification of the inventors of thepresent invention, when the molding shrinkage is less than 0.02%, asufficient adhesion force cannot be ensured between the electroformingportion 10 and the resin, resulting in anxiety about durability of thebearing. On the other hand, it has been found that, when the moldingshrinkage exceeds 2.0%, a shrinking force of the resin portion becomesexcessive, which adversely affects the surface accuracy of the bearing.

Note that it is also possible to use a metal material as the injectedmaterial. Metal injection molding includes molten metal injectionmolding and metal powder injection molding, and either of those can beadopted in the present invention. The former one is a technology inwhich metal chips and slugs are molten or semi-molten, then flown intothe metal mold, followed by the molding. In particular, when low meltingpoint metal such as a magnesium alloy or an aluminum alloy is used, amelting facility can be downsized. The latter one is a technology inwhich metal powder and a binder are mixed/kneaded together and caused toflow into the metal mold to be then taken out from the metal mold, anddegreased and sintered. This technology is called metal injectionmolding (MIM) in general. In the case of this MIM molding, the metal tobe used is not limited to the low melting point metal such as themagnesium alloy or the aluminum alloy, and other metal materials such asa copper alloy, an iron alloy, and a copper-iron alloy can be widelyselected according to the purpose of the bearing. As described above,the metal is used as an injection molding material, thus making itpossible to further-enhance the strength, the heat resistance, theconductivity, and the like as compared with the case of using the resinmaterial.

In addition to the above-mentioned resin material and the metalmaterial, for example, ceramics can be used. It is possible to use, forexample, so-called CIM molding or the like, in which a mixture ofceramic powder and the binder is injection molded, and then is degreasedand sintered. In this case, a feature is obtained that the injectedmaterial is lighter than the metal material and more excellent than theresin material in rigidity and heat resistance.

After the insert molding is completed, the mold is opened. Then, amolded article as shown in FIG. 3 is obtained, in which theelectroforming shaft 11 composed of the master shaft 12 and theelectroforming portion 10 and a mold portion 14 are integrated together.

The above-mentioned molded article is then conveyed to a separationstep, where the molded article is separated into a member (bearingmember 8) in which the electroforming portion 10 and the mold portion 14are integrated together and into the master shaft 12.

In the separation step, the bearing member 8 and the master shaft 12 areseparated from each other. To be specific, for example, by applying animpact to the electroforming shaft 11 or the bearing member 8, aresidual stress accumulated in the electroforming portion 10 isreleased, an inner circumferential surface 10 a of the electroformingportion 10 is expanded in a radius direction, and a gap (desirably,larger than a depth of the dynamic pressure grooves) is formed betweenthe inner circumferential surface 10 a of the bearing member 8 and theouter circumferential surface 12 a of the master shaft 12. By formingthe gap, the axial engagement of the irregularities between the radialbearing surfaces A formed on the inner circumferential surface of thebearing member 8 and the forming portion N formed on the outercircumferential surface 12 a of the master shaft 12 is cancelled. Hence,the master shaft 12 is drawn out from the bearing member 8 in the axialdirection after the electroforming portion 10 is peeled off from theouter circumferential surface 12 a of the master shaft 12 by applyingthe impact to the electroforming shaft 11 or the bearing member 8, thusmaking it possible to smoothly separate the master shaft 12 and thebearing member 8 from each other without damaging the radial bearingsurfaces A. Note that a diameter expansion amount of the electroformingportion 10 can be controlled within a range of 1 μm to several ten μm,for example, by changing the thickness of the electroforming portion 10.

When a sufficient diameter expansion amount cannot be ensured on theinner circumferential surface 10 a of the electroforming portion 10 onlyby releasing the stress, the electroforming portion 10 and the mastershaft 12 are heated or cooled, and a difference in thermal expansionamount is generated there between, thus also making it possible toseparate the master shaft 12 and the bearing member 8 from each other.

In this case, if the master shaft 12 is formed in advance of the metalmaterial or the ceramic material as described above, deformation of themaster shaft 12 can be avoided even under ahigh-temperature/high-pressure environment at the time of the injectionmolding. Hence, deformation of the forming portion N at the time of theinjection molding can be avoided, and accordingly, the radial bearingsurfaces A can be formed with high accuracy. Further, the master shaft12 separated from the electroforming portion 10 can be repeatedly usedfor the fabrication of the bearing member 8, and the radial bearingsurfaces A are formed into a shape corresponding to that of the formingportion N of the master shaft 12. Hence, fabrication cost of the mastershaft 12 can be suppressed, and in addition, the bearing member 8including the dynamic pressure generating portion, in which accuracy isless varied from others and high, can be mass-produced stably.

Note that an outer surface of the electroforming portion 10 is formedinto a rough surface owing to characteristics of the electroformingprocess. Accordingly, at the time of the insert molding, the materialconstructing the mold portion 14 enters micro irregularities on theouter surface of the electroforming portion, thereby exerting a strongadhesion force due to an anchor effect. Further, as described above, inthe present invention, the flanges 20 are formed on the electroformingportion 10, and the electroforming portion 10 is subjected to the insertmolding in a form of including the flanges 20 as well. Accordingly,detachment prevention and rotation prevention are effected between theelectroforming portion 10 and the mold portion 14. Hence, the adhesionforce between the electroforming portion 10 and the mold portion 14increases, thus making it possible to provide the bearing member 8having excellent durability and high reliability. In particular, in thecase of forming the flanges 20 by plastically deforming theelectroforming portion 10 as in the embodiment shown in FIGS. 4 and 5,as shown in FIG. 1, a shape of outer circumferential surfaces 20 a ofthe flanges 20 becomes a non-perfect circular shape having randomirregularities. As a result, a high effect of preventing the rotation isobtained. Note that, in FIG. 1, the irregularities of the outercircumferential surfaces 20 a are drawn in an exaggerated manner for thepurpose of facilitating the understanding.

As described above, in the case of forming the flanges 20 by the plasticdeformation, if pressurizing force from the metal mold, which is appliedto the electroforming portion 10, is too large, there is a risk that theinner circumferential surface of the electroforming portion 10, which isbrought into intimate contact with the master shaft 12, is peeled offfrom the outer circumferential surface of the master shaft 12 owing tothe impact at that time. When the electroforming portion 10 is peeledoff, at that moment, the electroforming portion 10 expands in diameter,and a gap is formed with the master shaft 12. Accordingly, at the timeof the subsequent injection molding, the inner circumferential surfaceof the electroforming portion 10 randomly shrinks in diameter due to aninjection pressure, and there is a risk that an accuracy decrease of theinner circumferential surface 10 a is brought. In order to prevent suchthe situation, it is necessary to make an effort to prevent theelectroforming portion 10 from being peeled off from the master shaft 12at the time before the injection molding, and this is considered to beachievable by controlling an upper limit of the amount of plasticdeformation of the electroforming portion 10.

According to examination from a viewpoint as described above, thefollowing has been found. In the bearing member 8 shown in FIG. 6, whenan axial length of the electroforming portion 10 (shown by solid linesin FIG. 6) after the plastic deformation is L1, and an axial length ofthe electroforming portion 10 (shown by broken lines in FIG. 5) beforethe plastic deformation is L2, if a change A (=L2−L1) in the axiallength of the electroforming portion 10 is within 50% (desirably, within20%) of the axial length L1 of the electroforming portion 10 after theplastic deformation, the electroforming portion 10 can be prevented frombeing peeled off owing to the plastic deformation before the injectionmolding. On the other hand, if A is equal to 0, the flanges 20 cannot beformed. Hence, it is desirable to determine L1 and L2 so as to satisfythe following expression:

0<A/L1≦0.5

In the above description, the case has been shown, where the flanges 20are formed by the plastic deformation; however, the flanges 20 can alsobe formed by other methods than the plastic deformation. For example, asshown in FIG. 7, if the master shaft 12 is formed in advance into ashaft shape with a step difference, when the master shaft 12 is immersedinto the electrolytic solution in the electroforming step, inclinedflanges 20 as shown in FIG. 7 can be formed on corner portions 12 a ofthe master shaft 12 after the end of the electroforming depending onelectroforming conditions. This is because, in general, a depositionamount of metal particles increases in the corner portions 12 a ascompared with that in other smooth portions of the master shaft 12.

Hence, if the electroforming portion 10 including the flanges 20 isformed by the injection molding (as shown by a chain double-dashed line)after the formation of the flanges 20, the effect to prevent theelectroforming portion 10 from being peeled off or being rotated can beobtained similarly to the above.

As shown in FIG. 8, on the inner circumferential surface (innercircumferential surface 10 a of the electroforming portion 10) of thebearing member 8 from which the master shaft 12 has been separated, thetwo radial bearing surfaces A composed of a plurality of dynamicpressure grooves 8 a 1 and 8 a 2 and the protruding portions fordefining the dynamic pressure grooves 8 a 1 and 8 a 2 are formedvertically apart from each other. As will be described later, the radialbearing surfaces A form a radial bearing gap with an outercircumferential surface of a shaft member 2 as the bearing member 8 isincorporated into the bearing device.

Next, an example of a fluid dynamic bearing device 1 using the bearingmember 8 fabricated in the above-mentioned steps is shown in FIG. 9. Asshown in FIG. 9, the fluid dynamic bearing device 1 includes in additionto the bearing member 8, as main constituent parts, a housing 7 whichhas a bottom portion 7 c on one end thereof, for fixing the bearingmember 8 to an inner circumference thereof, the shaft member 2 insertedinto the inner circumference of the bearing member 8, and a seal member9. Note that a description will be made below while defining that theseal member 9 side is an upper side and an axially opposite side to theseal member 9 side is a lower side for the sake of convenience of thedescription.

The housing 7 is formed into a closed-end cylindrical shape formed of ametal material such as stainless steel or brass or a resin material. Thehousing 7 includes an opening portion 7 a at one end, and the other endthereof is sealed. The housing 7 further includes a cylindrical sideportion 7 b, and the bottom portion 7 c on the opposite end side of theside portion 7 b with respect to the opening portion 7 a. In thisembodiment, the side portion 7 b and the bottom portion 7 c are formedseparately from each other, and the bottom portion 7 c is fixed to alower inner circumference of the side portion 7 b by means such asadhesion, press-fitting, and welding. Though not shown, on a partiallyannular region serving as a thrust bearing surface of the bottom portion7 c, a plurality of dynamic pressure grooves, for example, arranged in aspiral configuration or a herringbone configuration are formed as adynamic pressure generating portion. This type of dynamic pressuregrooves can be molded through a pressing process and the likesimultaneously with the molding of the bottom portion 7 c. Further, theside portion 7 b and the bottom portion 7 c can also be formedintegrally. Note that the materials for forming the side portion 7 b andthe bottom portion 7 c may be either the same or different from eachother as long as the materials can meet performance required therefor.

The shaft member 2 is formed, for example, of a metal material such asstainless steel separately from the above-mentioned master shaft 12. Theshaft member is composed of a shaft portion 2 a, and a flange portion 2b provided on one end of the shaft portion 2 a integrally or separately.An outer circumferential surface of the shaft portion 2 a forms aperfect circle in cross section, which does not have the dynamicpressure grooves or the like. The shaft member 2 can also be formed intoa hybrid structure composed of a metal portion and a resin portion, aswell as is formed only of the metal material (for example, the shaftportion 2 a is formed of a metal material, and the flange portion 2 b isformed of a resin material). An outer diameter dimension of the shaftportion 2 a is slightly smaller than an inner diameter dimension of theprotruding portions, which define and form the dynamic pressure grooves8 a 1 and 8 a 2, in the radial bearing surfaces A formed on the bearingmember 8. Thus, the radial bearing gap approximately in a range of 1 μmto several ten μm is formed between the outer circumferential surface ofthe shaft portion 2 a and the two radial bearing surfaces A.

To an inner circumference of the opening portion 7 a of the housing 7,the seal member 9 formed of a metal material such as brass or a resinmaterial is fixed by means such as the press-fitting and the adhesion.In this embodiment, the seal member 9 forms an annular shape, and isformed separately from the housing 7. An inner circumferential surface 9a of the seal member 9 is opposite to the outer; circumferential surfaceof the shaft portion 2 a through a seal space S having a predeterminedvolume. The outer circumferential surface of the shaft portion 2 a,which is opposite to the seal space S, is formed as a tapered surface 2a 2 gradually reduced in diameter as extending upward, and alsofunctions as a centrifugal seal when the shaft member 2 is rotated.After the fluid dynamic bearing device 1 is assembled, an inner space ofthe fluid dynamic bearing device 1 hermetically sealed by the sealmember 9 is filled with, for example, lubricating oil as lubricatingfluid, and in this state, an oil level of the lubricating oil ismaintained within the seal space S. Note that the seal member 9 can alsobe formed integrally with the housing 7 for the purpose of reducing thenumber of parts and the number of assembly man-hours.

The bearing member 8 is fixed to an inner circumferential surface of theside portion 7 b of the housing 7. With regard to a method of fixing thebearing member 8 to the inner circumference of the housing, fixing meanssuch as the press-fitting, the adhesion, a combination thereof, and thewelding is selected according to design conditions. The shaft member 2is rotatably inserted into the inner circumference of the bearing member8.

As described above, the bearing member 8 has a composite structurecomposed of the mold portion 14 formed of the resin material, and theelectroforming portion 10 fixed and attached to the innercircumferential surface of the mold portion 14, and the bearing 8 isformed into the cylindrical shape. The mold portion 14 and theelectroforming portion 10 are fixed and attached to each other with astrong force over the overall lengths in the axial direction, and theflanges 20 extending in the diameter direction are formed on the upperand lower end portions of the electroforming portion 10, thus detachmentprevention and rotation prevention are effected between the mold portion14 and the electroforming portion 10. The herringbone-shaped dynamicpressure grooves 8 a 1 and 8 a 2 are individually formed on the radialbearing surfaces A of the inner circumferential surface 8 a of thebearing member 8 by the above-mentioned electroforming process. In thisembodiment, the dynamic pressure groove 8 a 1 in the upper region isformed asymmetrically in the axial direction with respect to an axialcenter m (axial center of a region across upper and lower inclinedgrooves), and an axial dimension X1 of an upper region of the dynamicpressure groove 8 a 1 from the axial center m is larger than an axialdimension X2 of a lower region thereof. Therefore, feeding force(pumping force) of the lubricating oil by the dynamic pressure grooveswhen the shaft member 2 is rotated is relatively larger in the upperdynamic pressure groove 8 a 1 than in the lower symmetric dynamicpressure groove 8 a 2.

Further, though not shown, on a partially annular region serving as athrust bearing surface of a lower end surface Bc of the bearing member8, a plurality of dynamic pressure grooves arrayed, for example, in aspiral shape are formed as dynamic pressure generating portions. Thistype of dynamic pressure grooves can be molded simultaneously with theforming of the bearing member 8 as long as a groove shape is formed inadvance on a region of the lower mold 16, which is opposite to the lowerend surface 8 c. In this case, the lower mold 16 is the one to be usedin the molding step of forming the above-mentioned bearing member 8. Bythe simultaneous molding described above, a work of separately formingthe dynamic pressure grooves on the lower end surface 8 c can beomitted.

The fluid dynamic bearing device 1 is constructed as described above,and when the shaft member 2 is rotated, the two upper and lower regionsof the inner circumferential surface 8 a of the bearing member 8, whichserve as the radial bearing surfaces A, are individually opposite to anouter circumferential surface 2 a 1 of the shaft portion 2 a through theradial bearing gap. Then, as the shaft member 2 is rotated, dynamicpressure of the lubricating oil is generated in the above-mentionedradial bearing gap, and by the pressure, the shaft portion 2 a of theshaft member 2 is supported in a non-contact fashion with the bearingmember 8 rotatably in the radial direction. Thus, a first radial bearingportion R1 and a second radial bearing portion R2, which support thebearing member 2 rotatably in the radial direction in a non-contactfashion therewith, are formed.

Further, the region of the lower end surface 8 c of the bearing member8, which serves as the thrust bearing surface, is opposite to an upperend surface 2 b 1 of the flange portion 2 b through the thrust bearinggap, and a region of an upper end surface 7 c 1 of the bottom portion 7c, which serves as the thrust bearing surface, is opposite to a lowerend surface 2 b 2 of the flange portion 2 b through the thrust bearinggap. Then, as the shaft member 2 is being rotated, the dynamic pressureof the lubricating oil is also generated in the thrust bearing gaps. Byusing the pressure, the shaft member 2 is supported in a non-contactfashion with the housing 7 and the bearing member 8 rotatably in both ofthe thrust directions. Thus, a first thrust bearing portion T1 and asecond thrust bearing portion T2 for supporting the shaft member 2 insuch a non-contact fashion rotatably in both of the thrust directions,are formed.

Note that, while the shaft member 2 is being rotated, the lubricatingoil is forced into the bottom side of the housing 7. Accordingly, ifsuch the state continues, the pressure of the lubricating oil in thethrust bearing gaps of the thrust bearing portions T1 and T2 increasesto an extreme extent, so there is a fear that bubbles may be generatedin the lubricating oil, that the lubricating oil may leak, or thatvibrations may occur, all of which may result from the extreme increaseof the pressure. In this case, as shown in FIGS. 8 and 9, circulationpaths 8 d 1 and 9 b 1 which allow the thrust bearing gaps (inparticular, the thrust bearing gap of the first thrust bearing portionT1) and the seal space S to communicate with each other are provided onan outer circumferential surface 8 d of the bearing member 8 and a lowerend surface 9 b of the seal member 9, respectively. Then, thelubricating oil flows between the thrust bearing gaps and the seal spaceS through the circulation paths 8 d 1 and 9 b 1, and the pressuredifference is released early, and such harmful effects as describedabove can be prevented. As an example, FIG. 9 shows a case where thecirculation path 8 d 1 is formed on the outer circumferential surface 8d of the bearing member 8, and a case where the circulation path 9 b 1is formed on the lower end surface 9 b of the seal member 9. However,the circulation path 8 d 1 can also be formed on the innercircumferential surface of the housing 7, and the circulation path 9 b 1can also be formed on the upper end surface 8 b of the bearing member 8.

FIG. 10 shows a construction example of a spindle motor for aninformation apparatus, into which the fluid dynamic bearing device 1shown in FIG. 9 is incorporated. The spindle motor is the one to be usedfor a disk drive device such as an HDD, and includes the fluid dynamicbearing device 1 for rotatably supporting the shaft member 2 in anon-contact fashion, a rotor (disk hub) 3 attached to the shaft member2, and a stator coil 4 and a rotor magnet 5, both of which are opposedto each other through a radial gap. The stator coil 4 is attached to anouter circumference of a bracket 6, and the rotor magnet 5 is attachedto an inner circumference of the disk hub 3. The housing 7 of the fluiddynamic bearing device 1 is attached to an inner circumference of thebracket 6. On the disk hub 3, one or a plurality of disks D such asmagnetic disks are held. When the stator coil 4 is energized, the rotormagnet 5 is rotated by an electromagnetic force between the stator coil4 and the rotor magnet 5, and the disk hub 3 and the shaft member 2 arethereby rotated integrally with each other.

FIG. 11 shows a construction example of a fluid dynamic bearing device,and further, a motor, using the bearing member 8 of the embodiment shownin FIG. 6. The motor has a radial bearing portion R for supporting theshaft member rotatably in the radial direction, and a thrust bearingportion T for supporting the shaft member rotatably in the thrustdirection. The radial bearing portion R is constructed by inserting theshaft member 2 into the inner circumference of the bearing member 8, andthe thrust bearing portion T is constructed by supporting a convexspherical shaft end of the shaft member 2 by a thrust plate 24 opposedto an end surface of the bearing member 8 in a contact fashiontherewith.

In the disk drive device such as the HDD, there is a risk that the disksD are charged with static electricity by friction with the air, and thecharged static electricity is discharged instantaneously to peripheraldevices such as a magnetic head, thus adversely affecting the peripheraldevices. In particular, when the thrust plate 23 is made of the resin,or when the thrust bearing portion T is composed of the dynamic bearing,a tendency to bring such the adverse effect becomes significant. On theother hand, when the mold portion 14 is formed by the injection moldingof the metal material, the static electricity built up on the disks D isdischarged rapidly to a ground side through a route of the shaft member2 → the electroforming portion 4 → the mold portion 14 → a bracket 27.Hence, the disks D can be restricted from being charged with the staticelectricity, and a spark can be prevented from occurring between thedisks and the peripheral devices.

The above mentioned bearing member 8 can be used not only for the fluiddynamic bearing device 1 shown in FIG. 9 but also widely for fluiddynamic bearing devices with other constructions. A description will bemade below of the other constructions of the fluid dynamic bearingdevice with reference to the drawings. Common reference symbols will beassigned to members having the same constructions as and commonfunctions to those of the fluid dynamic bearing device 1 shown in FIG.9, and a repeated description will be omitted.

A fluid dynamic bearing device shown in FIG. 12 is the one, in which thethrust bearing portion T is disposed on the opening portion 7 a side ofthe housing 7, and supports the shaft member 2 in a non-contact fashionwith the bearing member 8 in one thrust direction. The flange portion 2b is provided above the lower end of the shaft member 2, and the thrustbearing gap of the thrust bearing portion T is formed between a lowerend surface 2 b 2 of the flange portion 2 b and the upper end surface 8b of the bearing member 8. The seal member 9 is attached to an innercircumference of the opening portion of the housing 7, and the sealspace S is formed between the inner circumferential surface 9 a of theseal member 9 and the outer circumferential surface of the shaft portion2 a of the shaft member 2. The lower end surface 9 b of the seal member9 is opposite to the upper end surface 2 b 1 of the flange portion 2 bthrough an intermediation of an axial gap. When the shaft member 2 isdisplaced upwards, the upper end surface 2 b 1 of the flange portion 2 bengages with the lower end surface 9 b of the seal member 9, therebypreventing the shaft member 2 from being drawn out.

FIG. 13 is a view showing another embodiment of the fluid dynamicbearing device 1. The fluid dynamic bearing device 1 shown in FIG. 13 isdifferent from the fluid dynamic bearing device 1 shown in FIG. 9 mainlyin the following points. To be specific, in FIG. 13, the seal space S isformed on an outer diameter portion of the housing 7, and the thrustbearing portion T2 is formed between the upper end surface of thehousing 7 and a lower end surface 3 a 1 of a plate portion 3 aconstructing the disk hub 3.

FIG. 15 is a view showing still another embodiment of the fluid dynamicbearing device 1. This embodiment is largely different from theembodiment shown in FIG. 9 in that a bearing member 28 is formedintegrally with the housing 7 in a form of including the housing 7. Interms of a structure thereof, as in the bearing member 8 shown in FIG.9, the bearing member 28 is also composed of the mold portion 14 formedof the resin material (metal material in some cases), and theelectroforming portion 10 fixed and attached to the innercircumferential surface of the mold portion 14. In terms of a shapethereof, the bearing member 28 is composed of a sleeve-like sleeveportion 28 a capable of inserting the shaft portion 2 a into an innercircumference thereof, a seal fixing portion 28 b extending upward froman outer diameter side of the sleeve portion 28 a and capable of fixingthe seal member 9 to an inner circumference of the fixing portion 28 bitself, and a bottom fixing portion 28 c extending downward from theouter diameter side of the sleeve portion 28 a and capable of fixing thebottom portion 7 c to an inner circumference of the bottom fixingportion 28 c itself. In the sleeve portion 28 a, an axial circulationpath 29 for allowing an upper end surface 28 a 2 and a lower end surface28 a 3 thereof to communicate with each other, is provided. In thisembodiment, the bearing member 28 is molded while including a portionserving as the housing in the molding step shown in FIG. 4. Accordingly,the number of parts and the number of assembly man-hours are reduced,thus making it possible to achieve cost reduction of the fluid dynamicbearing device 1.

FIG. 16 is a view showing yet another embodiment of the fluid dynamicbearing device 1. This embodiment is largely different from theembodiment shown in FIG. 13 in that, as in the embodiment shown in FIG.15, the bearing member 28 is formed integrally with the housing 7 in aform of including the housing 7 which is a separate body in FIG. 13.Also in this embodiment, the number of parts and the number of assemblyman-hours are reduced, thus making it possible to achieve the costreduction of the fluid dynamic bearing device 1.

As constructions of the radial bearing portions R1 and R2 and the thrustbearing portions T, T1 and T2, the above description shows theconstructions with which the fluid dynamic pressure is generated byusing the herringbone-shaped and the spiral-shaped dynamic pressuregrooves. However, the present invention is not limited to this.

For example, so-called multi-lobe bearing and step bearing may also beemployed as the radial bearing portions R1 and R2. In those bearings, aplurality of circular arc surfaces (in the multi-lobe bearing) and aplurality of axial grooves (in the step bearing) serve as the dynamicpressure generating portions for generating the dynamic pressures in theradial bearing gaps. Those dynamic pressure generating portions areformed on the electroforming portion 10 of the bearing member 8, and aforming method thereof conforms to the respective steps (refer to FIGS.2A to 2C and 5) in the case of forming the dynamic pressure grooves.Accordingly, a description thereof will be omitted.

FIG. 17 shows an example of the case where one or both of the radialbearing portions R1 and R2 are composed of the multi-lobe bearings. Inthis example, the region of the inner circumferential surface 8 a of thebearing member 8, which serves as the radial bearing surfaces, iscomposed of three circular arc surfaces 33 (this is a so-calledthree-lobe bearing). Centers of curvatures of the three circular arcsurfaces 33 are individually offset by equal distances from a shaftcenter O of the bearing member 8 (bearing portion 2 a). In regionsdefined by the three circular arc surfaces 33, the radial bearing gapsare wedge-like gaps 35 each of which is reduced gradually into wedgeshapes in both circumferential directions. Therefore, when the bearingmember 8 and the shaft portion 2 a are rotated relatively to each other,the lubricating oil in the radial bearing gap is forced into the minimumgap side of the wedge-like gaps 35 in response to a direction of suchrelative rotation, and the pressure thereof rises. By the dynamicpressure action of the lubricating oil, which is as described above, thebearing member 8 and the shaft portion 2 a are supported in anon-contact fashion. Note that deeper axial grooves called separatinggrooves may also be formed on boundary portions between the threecircular arc surfaces 33.

FIG. 18 shows another example of the case where one or both of theradial bearing portions R1 and R2 are composed of the multi-lobebearings. Also in this example, a region of the inner circumferentialsurface 8 a of the bearing member 8, which serves as the radial bearingsurfaces A, is composed of the three circular arc surfaces 33 (this isthe so-called three-lobe bearing). However, in the regions defined bythe three circular arc surfaces 33, the radial bearing gaps arewedge-like gaps 35 each of which is reduced gradually into wedge shapesin one circumferential direction. The multi-lobe bearing with such theconstruction is sometimes called a tapered bearing. Further, deeperaxial grooves called separating grooves 34 are formed on boundaryportions between the three circular arc surfaces 33. Therefore, when thebearing member 8 and the shaft portion 2 a rotates relatively to eachother in a predetermined direction, the lubricating oil in the radialbearing gap is forced into the minimum gap side of the wedge-like gaps35, and the pressure thereof rises. By the dynamic pressure action ofthe lubricating oil, which is as described above, the bearing member 8and the shaft portion 2 a are supported in a non-contact fashion.

FIG. 19 shows still another example of the case where one or both of theradial bearing portions R1 and R2 are composed of the multi-lobebearings. In this example, in the construction shown in FIG. 18, each ofpredetermined regions 9 on the minimum gap side of the three circulararc surface 33 is composed of a circular arc concentric with the bearingmember 8 (shaft portion 2 a) by taking the shaft center O thereof as thecenter of curvature. Hence, in the each predetermined region θ, theradial bearing gap (minimum bearing gap) becomes constant. Themulti-lobe bearing with such the construction is sometimes called ataper flat bearing.

FIG. 20 shows an example of a case where one or both of the radialbearing portions R1 and R2 are composed of the step bearings. In thisexample, in the region of the inner circumferential surface 8 a of thebearing member 8, which serves as the radial bearing surfaces, aplurality of axial groove-shaped dynamic pressure grooves 36 areprovided at a predetermined interval in the circumferential direction.

The multi-lobe bearings in the above-mentioned respective examples arethe so-called three-lobe bearings. However, without being limited tothis, a so-called four-lobe bearing, five-lobe bearing, and a multi-lobebearing composed of circular arc surfaces of which circular arcs are sixor more, may also be employed. Further, when the radial bearing portionis composed of the step bearing or the multi-lobe bearing, it ispossible to adopt a construction in which a single radial bearingportion is provided over the upper and lower regions of the innercircumferential surface 8 a of the bearing member 8, as well as aconstruction in which the two radial bearing portions are providedaxially apart from each other like the radial bearing portions R1 andR2. Those dynamic pressure generating portions are formed of theelectroforming portion 10 of the bearing member 8, and a forming methodthereof conforms to the respective steps (refer to FIGS. 2A to 2C) inthe case of forming the dynamic pressure grooves. Accordingly, adescription thereof will be omitted.

Further, with regard to forms of the thrust bearing portions T, T1 andT2, the constructions with which the dynamic pressure action of thelubricating oil is generated by the spiral-shaped dynamic pressuregrooves have been shown. However, such the thrust bearing portion canalso be composed of a so-called step bearing in which a plurality ofdynamic pressure grooves with a radial groove shape are provided in theregion serving as the thrust bearing surface, of a so-called wave-shapedbearing (in which the step shape turns to a wave shape), and the like(not shown).

The above-mentioned embodiments show that the fluid dynamic bearingdevice 1 is composed of the dynamic bearing which supports the shaftmember 2 in a non-contact fashion therewith in the thrust direction.However, the fluid dynamic bearing device 1 shown in FIG. 14 is composedof a pivot bearing which supports the shaft member 2 in acontact-fashion therewith in the thrust direction. In this case, a lowerend 2 a 3 of the shaft portion 2 a of the shaft member 2 is formed intoa convex spherical shape, and the lower end 2 a 3 is supported in acontact fashion by an upper end surface 24 a of the thrust plate 24fixed to the upper end surface 7 c 1 of the bottom portion 7 c of thehousing 7 by means such as adhesion.

Not only the radial bearing surfaces but also thrust bearing surfacescan be formed on the electroforming portion 10. A description will bemade below of an embodiment of this case with reference to FIGS. 21 to32.

A bearing member 8 having a construction of the present invention, whichis shown in FIG. 21, is fabricated through a step of fabricating amaster shaft 12 (refer to FIG. 22A), a step of masking a spot of themaster shaft 12, which requires the masking (refer to FIG. 22B), a stepof forming an electroforming shaft 11 by performing the electroformingprocess for an unmasked portion N of the master shaft 12 (refer to FIG.22C), a step of forming a bearing member 8 by molding an electroformingportion 10 of the electroforming shaft 11 with a resin and the like(refer to FIG. 25), and a step of separating the electroforming portion10 and the master shaft 12 from each other.

The above-mentioned steps shown in FIGS. 22A to 22C are basically commonto the steps shown in FIGS. 2A to 2C, and accordingly, common matterswill be omitted below, and a description will be mainly made below ofdifferent matters.

In the masking step shown in FIG. 22B, the masking 13 (shown by ascatter pattern) is applied to an upper portion of an outercircumferential surface of the master shaft 12 and to an upper endsurface thereof. On such the portions (masked portion M) subjected tothe masking, electroforming metal is not electrolytically deposited atthe time of an electroforming process to be described later, and theelectroforming portion 10 is not formed. Meanwhile, the masking is notapplied to the outer circumferential surface and lower end surface ofthe master shaft 12, except for the masked portion M, and such theportions (unmasked portion N) which are not subjected to the maskingbecome forming portions for forming an inner circumferential surface(radial bearing surfaces A) of the electroforming portion 10 and aninner bottom surface (thrust bearing surface B) thereof at the time ofthe electroforming process.

As shown in FIGS. 22A and 22B, in the unmasked portion N of the mastershaft 12, on the outer circumferential surface of the master shaft 12, aradial bearing surface forming portion N1 having an irregular shapecorresponding to a dynamic pressure groove pattern of the radial bearingsurfaces A is formed. Aspects of irregularities of the radial bearingsurfaces A and the radial bearing surface forming portion N1 arecompletely opposite to each other, and protruding portions of the radialbearing surfaces A correspond to recessed portions 12 a 1 and 12 a 2 ofthe radial bearing surface forming portion N1. The shown exampleillustrates the case where the recessed portions 12 a 1 and 12 a 2 ofthe radial bearing surface forming portion N1 correspond to theherringbone-shaped dynamic pressure groove pattern. However, therecessed portions 12 a 1 and 12 a 2 can also be formed into a shapecorresponding to the spiral-shaped dynamic pressure groove pattern.

In a similar way to the above, in the unmasked portion N, on a partiallyannular region of a lower end surface 12 c of the master shaft 12, asshown in FIG. 23, a thrust bearing surface forming portion N2 having anirregular shape corresponding to a dynamic pressure groove pattern ofthe thrust bearing surface B to be described later is formed. Also onthe thrust bearing surface forming portion N2, an aspect ofirregularities thereof is completely opposite to that of the thrustbearing surface B. The shown example illustrates the case where thethrust bearing surface forming portion N2 corresponds to a spiral-shapeddynamic pressure groove pattern. However, the thrust bearing surfaceforming portion N2 can also be formed into a shape corresponding to apattern of a herringbone-shaped dynamic pressure groove.

By being subjected to the steps described above, as shown in FIG. 22C,the electroforming shaft 11 is formed, in which the closed-endcylindrical electroforming portion 10 is coated on the unmasked region Nof the outer circumferential surface 12 a and lower end surface 12 c ofthe master shaft 12. At this time, as shown in FIG. 24, onto an innercircumferential surface 10 a of the electroforming portion 10, a shapeof the radial bearing surface forming portion N1 of the outercircumferential surface 12 a of the master shaft 12 is transferred, andthe radial bearing surfaces A having a plurality of dynamic pressuregrooves 8 a 1 and 8 a 2 are formed apart from each other in the axialdirection. Further, onto an inner bottom surface 10 c of theelectroforming portion 10, a shape of the thrust bearing surface formingportion N2 of the lower end surface 12 c of the master shaft 12 istransferred, and the thrust bearing surface B having the plurality ofdynamic pressure grooves is formed (not shown).

Next, the electroforming portion 11 is conveyed to a molding step shownin FIG. 25, and injection molding (insert molding) using a resinmaterial is performed while taking the electroforming shaft 11 as aninsert part.

When the mold is opened after solidifying the resin material, as shownin FIG. 24, a molded article is obtained, in which the electroformingshaft 11, composed of the master shaft 12 and the electroforming portion10, and a mold portion 14 are integrated together.

The above-mentioned molded article is then conveyed to a separationstep, where the molded article is separated into the one (bearing member8), in which the electroforming portion 10 and the mold portion 14 areintegrated together, and the master shaft 12.

As shown in FIG. 27, the bearing member 8 separated from the mastershaft 12 forms a closed-end cylindrical shape having a side portion 8 band a bottom portion 8 c which are integral with the bearing member 8.In particular, in this embodiment, an upper end of the electroformingportion 10 is also covered with the mold portion 14. Accordingly, theelectroforming portion 10 can be prevented from being drawn outtherefrom. An inner circumferential surface of such the coating portionof the mold portion 14 is formed as a tapered surface 14 a, and a sealspace is formed between the tapered surface 14 a and the outercircumferential surface of the shaft member 2 after the bearing deviceis assembled as will be described later.

Into an inner circumference of the shaft member 8 separated from themaster shaft 12, as shown in FIG. 27, a shaft member 2 fabricatedseparately from the master shaft 12 is inserted, and a fluid dynamicbearing device (fluid dynamic bearing device) 1 is thus constructed. Theshaft member 2 is formed of a metal material rich in abrasion resistancesuch as stainless steel, an outer circumferential surface 2 a thereof isformed into a perfect circular shape which does not have the dynamicpressure grooves, and a lower end surface 2 b thereof is formed into aflat surface shape which does not have the dynamic pressure grooves. Anouter diameter dimension of the shaft member 2 is slightly smaller thanan inner diameter dimension of regions of the radial bearing surfaces Abetween the dynamic pressure grooves, which are the protruding portionswhich define the dynamic pressure grooves. Thus, a radial bearing gap(not shown) approximately in a range of 1 μm to several ten μm is formedbetween the outer circumferential surface of the shaft member 2 and thetwo radial bearing surfaces A.

Further, by inserting the shaft member 2 into the inner circumference ofthe bearing member 8, the tapered seal space S is formed between thetapered surface 14 a of an upper end opening portion of the mold portion14 and the outer circumferential surface 2 a of the shaft member 2.After the insertion of the shaft member 2, an inner space of the fluiddynamic bearing device 1 hermetically sealed at the seal space S isfilled with, for example, the lubricating oil as the lubricating fluid.In this state, an oil level of the lubricating oil is maintained withinthe seal space S. The seal space S can be formed into a cylindricalspace having the same width overall, as well as is formed into thetapered space in which an upper space is expanded. Further, the taperedsurface 14 aconstructing the seal can also be composed of a separatemember from the mold portion 14.

The fluid dynamic bearing device 1 is constructed as described above.When the shaft member 2 and the bearing member 8 rotate relatively toeach other (for example, when the shaft member 2 rotates), the dynamicpressure of the lubricating oil is generated in the above-mentionedradial bearing gap, and by the pressure thereof, the shaft member 2 issupported rotatably in the radial direction in a non-contact fashionwith the bearing member 8. Thus, a first radial bearing portion R1 and asecond radial bearing portion R2, for supporting the bearing member 2rotatably in the radial direction in a non-contact fashion therewith,are formed.

Further, the thrust bearing surface B of the bearing member 8 isopposite to the lower end surface 2 b of the shaft member 2 through athrust bearing gap. As the shaft member 2 rotates, the dynamic pressureof the lubricating oil is also generated in the thrust bearing gap, andby the pressure thereof, the shaft member 2 is supported in anon-contact fashion with the bearing member 8 rotatably in the thrustdirection. Thus, a thrust bearing portion T is formed, for supportingthe shaft member 2 in the non-contact fashion rotatably in the thrustdirection.

As described above, in the fluid dynamic bearing device 1 of the presentinvention, both of the radial bearing surfaces A and the thrust bearingsurface B are formed on the electroforming portion 10, and the bearingmember 8 is formed by the injection molding in which each electroformingportion 10 is inserted. Therefore, the constructions of the radialbearing portions R1 and R2 and the thrust bearing portion T can besimplified, and in addition, the number of parts and the number ofman-hours can be reduced, thus making it possible to achieve the costreduction of the bearing device 1. Further, since the radial bearingsurfaces A and the thrust bearing surface B are subjected to theelectroforming process, the dynamic pressure grooves with high accuracycan be formed, and high bearing performance can be obtained. Inaddition, powder that may be generated by cutting is not generated asthe bearing surfaces A and B are being molded, and a problem ofcontamination is also solved.

Further, the master shaft 12 fabricated once can be repeatedly used, andthe radial bearing surfaces A and the thrust bearing surface B afterbeing molded are formed into shapes corresponding to the surface shapesof the forming portions N1 and N2 of the master shaft. Hence, thebearing member 8 having little variations in accuracies of the dynamicpressure grooves among each other can be obtained, thus making itpossible to stably mass-produce the fluid dynamic bearing device 1having high rotational accuracy.

Note that an outer surface of the electroforming portion 10 is formedinto a rough surface owing to characteristics of the electroformingprocess. Accordingly, at the time of the insert molding, the resinmaterial constructing the mold portion 14 enters into spaces caused bymicro irregularities on the outer surface of the electroforming portion10, and exerts an anchor effect. Therefore, a strong adhesion force isexerted between the electroforming portion 10 and the mold portion 14,and the electroforming portion 10 and the mold portion 14 are reliablyprevented from being rotated with respect to each other and detachedfrom each other. Hence, it becomes possible to provide the strongbearing member 8 rich in impact resistance.

When an effect of preventing such rotation and drawing out isinsufficient, as shown in FIG. 28, a flange 20 is formed on theelectroforming portion integrally therewith, and is incorporated in themold portion 14. Then, the effect of preventing the rotation and thedrawing out can be further enhanced.

In the shown example, the flange 20 is formed in an inclined manner on acorner portion of the radial bearing surface A and the thrust bearingsurface B, and it is possible to form this type of flange 20 in theelectroforming process. To be specific, when the master shaft 12 of theshown embodiment is immersed into the electrolytic solution, usually, adeposition amount of metal particles is large in a lower end cornerportion 12 d of the master shaft 12 as compared with other portions.Accordingly, the inclined flange 20 shown in FIG. 28 grows. Therefore,when the electroforming shaft 11 added with the flanges 20 is formed asit is by the resin material, it becomes possible to use the flange 20 asa portion for preventing the rotation and the drawing out.

Note that the flange 20 can also be formed by plastically deforming theelectroforming portion 10. In this case, a forming position of theflange 20 is not particularly limited, and for example, the flange 20may also be formed by plastically deforming the upper end of theelectroforming portion 10 to the outer diameter side.

The above description illustrates the case of forming the dynamicpressure grooves symmetrically in the axial direction on the radialbearing portions R1 and R2. However, the dynamic pressure grooves canalso be formed asymmetrically in the axial direction. FIG. 29 shows astate where the shaft member 2 is drawn out from the bearing member 8,where an example of the above-mentioned asymmetric forming is shown. InFIG. 29, the dynamic pressure groove 8 a 1 is formed asymmetrically inthe radial direction with respect to an axial center thereof (axialcenter of the region between the upper and lower inclined grooves) bythe upper radial bearing portion R1, and an axial dimension X1 of anupper region of the dynamic pressure groove 8 a 1 from the axial centerm is set larger than an axial dimension X2 of a lower region thereof. Onthe lower radial bearing portion R2, the dynamic pressure groove 8 a 2is formed symmetrically in the axial direction, and axial dimensions ofupper and lower regions thereof are individually equal to theabove-mentioned axial dimension X2. In this case, the feeding force(pumping force) of the lubricating oil by the dynamic pressure grooveswhen the shaft member 2 is rotated is relatively larger in the upperdynamic pressure groove 8 a 1 than in the lower symmetric dynamicpressure groove 8 a 2. Therefore, a downward flow of the lubricating oilis generated in the radial bearing gap, thus making it possible tosupply the lubricating oil abundantly to the thrust bearing portion T.

Further, the above description illustrates the case of forming theradial bearing surfaces A and the thrust bearing surface B on theelectroforming portion 10 formed as a single member. However, aconstruction can also be adopted, in which the electroforming portion 10is divided into two or more portions, and both of the bearing surfaces Aand B are individually formed on electroforming portions thus formedseparately.

Next, a description will be made of an example of a motor, in which thefluid dynamic bearing device 1 described above is incorporated, withreference to the drawings.

FIG. 26 shows a construction example of the spindle motor for theinformation apparatus. The spindle motor is the one to be used for thedisk drive device such as the HDD, and includes the fluid dynamicbearing device 1 which rotatably supports the shaft member 2 in anon-contact fashion, a rotor (disk hub) 3 attached to the shaft member2, and a stator coil 4 and a rotor magnet 5, both of which are opposedto each other through an intermediation of a radial gap. The stator coil4 is attached to an outer circumference of a bracket 6, and the rotormagnet 5 is attached to an inner circumference of the disk hub 3. Thebearing member 8 of the fluid dynamic bearing device 1 is attached to aninner circumference of the bracket 6. On the disk hub 3, one or aplurality of disks D such as the magnetic disks are held. When thestator coil 4 is energized, the rotor magnet 5 is rotated byelectromagnetic force between the stator coil 4 and the rotor magnet 5,and the disk hub 3 and the shaft member 2 are thereby rotated integrallywith each other.

The construction of the present invention can be used not only for theabove-mentioned fluid dynamic bearing device 1 but also preferably forfluid dynamic bearing devices with a form shown below. A descriptionwill be made below of the construction of the fluid dynamic bearingdevice with reference to the drawings. Common reference symbols will beassigned to members having the same constructions as and commonfunctions to those of the fluid dynamic bearing device 1 shown in FIG.27, and a repeated description will be omitted.

FIG. 30 is a view showing another embodiment of the fluid dynamicbearing device 1. In the fluid dynamic bearing device 1, the dynamicpressure grooves 8 a 1 and 8 a 1 serving as the dynamic pressuregenerating portions are formed on the outer circumferential surface 2 aof the shaft member 2 and the lower end surface 2 b of the shaft member2 (the dynamic pressure groove formed on the lower end surface 2 b isnot shown), and the radial bearing surfaces A and the thrust bearingsurface B of the bearing member 8 are formed into a perfect circularshape in cross section and a flat surface shape, respectively, both ofwhich do not have the dynamic pressure grooves. In this case, the outercircumferential surface 12 a and lower end surface 12 c of the mastershaft 12 are formed into a perfect circular shape and a flat surfaceshape in cross section, respectively, both of which do not have thedynamic pressure grooves. The above-mentioned electroforming process andthe molding step are performed using the master shaft 12. Further, themaster shaft 12 is separated from the shaft bearing member 8 to form theradial bearing surfaces A and the thrust bearing surface B. Then theshaft member 2, which is a member different from the master shaft 12, isinserted into the inner circumference of the bearing member 8. Thedynamic pressure grooves are preliminarily formed on the outercircumferential surface 2 a and the lower end surface 2 b of the shaftmember 2 by means of a mechanical process or etching.

FIG. 31 is a view showing still another embodiment of the fluid dynamicbearing device 1. In the fluid dynamic bearing device 1, unlike theembodiments shown in FIGS. 27 and 29, the thrust bearing portion T iscomposed of a pivot bearing, and each of the radial bearing portions R1and R2 is composed of a perfect circular bearing which does not have thedynamic pressure generating portion. On the lower end of theshaft-member 2, a convex spherical surface 2 c is formed, and thespherical surface 2 c is supported by the thrust bearing surface portionB with the flat surface shape in a contact fashion therewith, and thethrust bearing portion T composed of the pivot bearing is thusconstructed. Further, the outer circumferential surface 2 a of the shaftmember 2 has the perfect circular shape in cross section, which does nothave the dynamic pressure grooves, and the perfect circular bearings arecomposed of the outer circumferential surface 2 a and the radial bearingsurfaces A with the perfect circular shape in cross section. In thiscase, any one of the radial bearing portions R1 and R2 and the thrustbearing portion T can also be replaced by the dynamic bearing shown inFIG. 27 or 29.

In the case of the embodiment shown in FIG. 31, as the shaft member 2,the master shaft 12 itself can also be used, as well as a memberseparate from the master shaft 12 as in the embodiments shown in FIGS.27 and 29. In this case, as shown in FIG. 32, the thrust bearing surfaceforming portion N2 with the flat surface shape, which forms the thrustbearing surface B, is formed on one end (upper end in FIG. 32) of themaster shaft 12, and the bearing constructing portion 2 c with theconvex spherical shape, which constructs the thrust bearing portion T,is formed on the other end of the master shaft 12. In the electroformingprocess, the electroforming portion 10 is formed on the thrust bearingsurface forming portion N2 of the master shaft 12 shown in FIG. 32, andmeanwhile, the masking is applied to the bearing constructing portion 2c, thereby forming the electroforming shaft 11. Next, the electroformingshaft 11 is subjected to the insert molding, and the bearing shaftmember 8 and the master shaft 12 are separated from each other. Afterthat, the master shaft 12 is reversed, and the spherical surface 2 cserving as the bearing constructing portion is inserted into the innercircumference of the bearing member 8, and then the spherical surface 2c is brought into contact with the thrust bearing surface B. Thus, thethrust bearing portion T composed of the pivot bearing is constructed.Thus, it becomes possible to concurrently use the master shaft 12 as ajig for molding the electroforming portion 10 and a constituent elementof the bearing device 1.

This method is also applied to the embodiments shown in FIG. 27 and FIG.29, thus making it possible to directly use the master shaft 12 as theshaft member 2. In this case, the flat surface is formed on one endsurface of the master shaft 12, and the dynamic pressure grooves (or amolding mold of the dynamic pressure grooves) is formed on the other endsurface. One of both end surfaces of the master shaft 12 becomes theforming portion of the thrust bearing surface B, and the other becomes abearing constructing portion for constructing the thrust bearing portionT.

The embodiments shown in FIGS. 27 and 29 illustrate the constructions ofthe radial bearing portions R1 and R2 for generating a fluid dynamicpressure by the herringbone-shaped and spiral-shaped dynamic pressuregrooves. However, the present invention is not limited to this, and forexample, the multi-lobe bearing and the step bearing, which are shown inFIGS. 17 to 20, may also be employed as the radial bearing portions R1and R2.

Further, the construction described above can also be applied to thecase where the perfect circular bearing which does not have the dynamicpressure generating portion is employed as the radial bearing portionsR1 and R2.

As shown in FIG. 33, fabrication steps of the perfect circular bearingare basically common to the steps shown in FIGS. 2A to 2C except thatthe outer circumferential surface of the master shaft 12 is of acylindrical surface shape which does not have the forming portion N. Tobe specific, the perfect circular bearing is fabricated through a stepof coating regions of the master shaft 12, which require the masking,with a masking material 13 (refer to FIG. 34), a step of forming theelectroforming shaft 11 by performing the electroforming process for anunmasked portion (refer to FIG. 35), and a step of forming the bearingmember 8 by molding the electroforming portion 10 of the electroformingshaft 11 by a resin and the like (refer to FIG. 5), and a step ofseparating the electroforming portion 10 and the master shaft 12 fromeach other.

When the radial bearing portions R1 and R2 are composed of the perfectcircular bearings, a size of the minimum clearance between the innercircumferential surface 10 a (bearing surface) of the electroformingportion 10 and the outer circumferential surface 2 a of the shaft member2 largely affects the bearing performance. As shown in FIG. 36, theminimum clearance δr is represented by a difference r1−r2 between aradius r1 of a virtual circle P1 inscribed to the bearing surface 10 aof the electroforming portion 10 and a radius r2 of a virtual circle P2circumscribed to the outer circumferential surface 2 a of the shaftmember 2. It is desirable that the minimum clearance δr be set as: δr≧0in a state where the shaft member 2 is inserted into the innercircumference of the bearing member 8 as shown in FIG. 36.

As described above, by ensuring the positive minimum clearance δr, anoccurrence of an unpreferable contact state between the shaft member 2and the bearing member 8 can be avoided when both thereof are rotatedrelatively to each other, and a stable rotation supporting state can bemaintained. The size of the minimum clearance δr mainly depends on thediameter expansion amount of the electroforming portion 10 in theseparation step. Accordingly, the thickness and electroformingconditions of the electroforming portion 10 are set so as to obtain suchthe numeric value as described above for the minimum clearance δr.

In this case, the sum of circularity (radius difference |r1−r3| betweenthe inscribed and circumscribed circles P1 and P3 of the bearing surface10 a in FIG. 36) of the bearing surface 10 a of the electroformingportion 10 and circularity (radius difference |r2−r4| between theinscribed and circumscribed circles P2 and P4 of the outercircumferential surface 2 a in FIG. 36) of the outer circumferentialsurface 2 a of the shaft member 2 is set to 4 μm or less.

As described above, the sum of the circularity of the bearing surface 10a and the circularity of the outer circumferential surface 2 a of theshaft member 2, which is opposite thereto, is restricted to be 4 μm orless, and thus the bearing gap between both of the surfaces 10 a and 2 abecomes more even with smaller variations over the circumferentialdirection. Hence, a more stable rotation supporting state can beobtained, and in addition, both of the surfaces 10 a and 2 a can beformed into relatively smooth surfaces, thus making it possible torestrict the abrasion thereof as much as possible when the shaft member2 and the bearing member 8 rotates relatively to each other. The outercircumferential surface of the master shaft 12 is subjected to afinishing process so as to satisfy the above-mentioned numericalconditions (δr≧0, |r1−r3|+|r2−r4|≦4 μm).

As an example, a master shaft with a shaft diameter Ø of 1.5 mm and acircularity of 0.5 μm was fabricated of stainless steel (SUS420F), andthe electroforming process was performed therefor for two hours by asulfamic acid nickel-bath, thereby forming the electroforming portion 10with a thickness of 0.1 mm. Then, it has been found that a bearingdevice which satisfies the above-mentioned numerical conditions can beobtained.

Note that, also when the radial bearing portions R1 and R2 are composedof the dynamic bearings as shown in FIGS. 8, 9, and the like, it isdesirable that the above-mentioned numerical conditions be satisfied. Inthis case, the minimum clearance or is represented by a differencebetween a radius of a virtual circle inscribed to the protrudingportions which define and form the dynamic pressure grooves and a radiusof a virtual circle circumscribed to the outer circumferential surface 2a of the shaft member 2. Further, the circularity of the bearing surface10 a is evaluated by an inner circumferential surface of the protrudingportions.

The perfect circular bearing can be used not only as a bearing devicefor rotation but also as a bearing device for sliding, a bearing devicefor rotation/sliding, and further, a bearing device for swinging, and itis possible to apply the construction of the present invention to all ofthose varieties of bearing devices. The “bearing device for rotation”means a device for supporting the relative rotation between the shaft 2and the bearing member 8, and the “bearing device for sliding” means adevice for supporting a relative linear motion between the shaft 2 andthe bearing member 8. The “bearing for rotation/sliding” means a devicewhich concurrently has functions of the above-mentioned two bearingdevices, for supporting both of the rotational motion and the linearmotion between the shaft 2 and the bearing member 8. The “bearing forswinging” means a bearing for supporting a swinging motion between theshaft 2 and the bearing member 8. In either cases, no problem occurswhich ever the bearing member 8 may be situated on the movable side orthe fixed side. Further, the bearing device can also be used bysupplying the lubricant such as oil to the bearing gap, as well as isused by supplying no oil thereto.

In the bearing device for rotation, a lateral cross section of themaster shaft 12 is basically formed into a circular shape. However, inthe case of the bearing device for sliding, a lateral cross section ofthe master shaft 12 can be formed into an arbitrary shape such as apolygonal shape and a non-perfect circular shape as well as the circularshape. Further, in the bearing device for sliding, the lateralcross-sectional shape of the master shaft 12 is basically constant inthe axial direction. However, in the bearing device for rotation and thebearing device for rotation/sliding, a form is sometimes adopted, inwhich the lateral cross-sectional shape is not constant over the overallaxis of the shaft.

In the fluid dynamic bearing device 1, the separate member fabricatedseparately from the master shaft 12 and with accuracy approximate tothat of the master shaft 12 is basically used as the shaft member 2.However, in the perfect circular bearing, in addition to the above, themaster shaft 7 can be directly used as the shaft member 2. In this case,the surface accuracy of the bearing surfaces 4 correspond to theaccuracy of the outer circumferential surface of the master shaft 7.Accordingly, a merit can be obtained that a matching work to beperformed after that becomes unnecessary.

In the above description, the lubricating oil has been shown as thelubricating fluid filled in the inside of the fluid dynamic bearingdevice. However, in addition to the above, fluid capable of generatingthe dynamic pressure in each of the bearing gaps, for example, gas suchas air can also be used as well as magnetic fluid.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a perspective view showing a bearing member according to thepresent invention;

FIG. 2A is a perspective view of a master shaft, FIG. 2B is aperspective view showing a state where masking is performed for themaster shaft, and FIG. 2C is a perspective view showing anelectroforming shaft;

FIG. 3 is a cross-sectional view of the bearing member immediately afterinsert molding;

FIG. 4 is a cross-sectional view of an injection molding metal mold;

FIG. 5 is across-sectional view of the injection molding metal mold;

FIG. 6 is a cross-sectional view of the bearing device;

FIG. 7 is a cross-sectional view showing an electroforming step;

FIG. 8 is a cross-sectional view of the bearing member;

FIG. 9 is a cross-sectional view showing an embodiment of a fluiddynamic bearing device;

FIG. 10 is across-sectional view showing an example of a spindle motorusing the fluid dynamic bearing device of the present invention;

FIG. 11 is a cross-sectional view showing another example of the spindlemotor using the fluid dynamic bearing device of the present invention;

FIG. 12 is a cross-sectional view showing another embodiment of thefluid dynamic bearing device;

FIG. 13 is a cross-sectional view showing still another embodiment ofthe fluid dynamic bearing device;

FIG. 14 is a cross-sectional view showing another embodiment of thefluid dynamic bearing device.

FIG. 15 is a cross-sectional view showing still another embodiment ofthe fluid dynamic bearing device;

FIG. 16 is a cross-sectional view showing yet another embodiment of thefluid dynamic bearing device;

FIG. 17 is a cross-sectional view showing another embodiment of a radialbearing portion;

FIG. 18 is a cross-sectional view showing another embodiment of theradial bearing portion;

FIG. 19 is a cross-sectional view showing still another embodiment ofthe radial bearing portion;

FIG. 20 is a cross-sectional view showing yet another embodiment of theradial bearing portion;

FIG. 21 is a perspective view of a bearing member according to thepresent invention;

FIG. 22A is a perspective view of a master shaft, FIG. 22B is aperspective view showing a state where masking is performed for themaster shaft, and FIG. 22C is a perspective view showing anelectroforming shaft;

FIG. 23 is a plan view showing a shaft end of the master shaft;

FIG. 24 is a cross-sectional view of the bearing member immediatelyafter insert molding;

FIG. 25 is a cross-sectional view showing a molding step;

FIG. 26 is a cross-sectional view showing an example of the spindlemotor using the fluid dynamic bearing device of the present invention;

FIG. 27 is a cross-sectional view showing an embodiment of the fluiddynamic bearing device;

FIG. 28 is a partially enlarged cross-sectional view of the bearingmember;

FIG. 29 is a cross-sectional view of the bearing member;

FIG. 30 is a cross-sectional view showing another embodiment of thefluid dynamic bearing device;

FIG. 31 is a cross-sectional view showing another embodiment of thefluid dynamic bearing device;

FIG. 32 is a front view of a master shaft to be used as a shaft member;

FIG. 33 is a perspective view of the mater shaft;

FIG. 34 is a perspective view showing the master shaft which issubjected to the masking;

FIG. 35 is a perspective view of an electroforming shaft; and

FIG. 36 is a cross-sectional view of the bearing device in a radiusdirection, and an enlarged view of the bearing device.

DESCRIPTION OF REFERENCE CHARACTERS

-   1 Fluid dynamic bearing device-   2 shaft member-   3 disk hub-   4 stator coil-   5 rotor magnet-   7 housing-   8 bearing member-   8 a 1 dynamic pressure grooves-   8 a 2 dynamic pressure grooves-   10 electroforming portion-   11 electroforming shaft-   12 master shaft-   13 masking-   14 mold portion-   20 flange-   A radial bearing surface-   N forming portion-   R1 first radial bearing portion-   R2 second radial bearing portion-   T thrust bearing portion-   T1 first thrust bearing portion-   T2 second thrust bearing portion

1. A fluid dynamic bearing device, comprising: a bearing member; and ashaft member inserted into an inner circumference of the bearing member,wherein the bearing member is formed by injection molding in which anelectroforming portion is inserted, and any one of an innercircumferential surface of the electroforming portion and an outercircumferential surface of the shaft member, which is opposed to theinner circumferential surface of the electroforming portion is providedwith a dynamic pressure generating portion formed thereon.
 2. A fluiddynamic bearing device according to claim 1, wherein the dynamicpressure generating portion is formed on the inner circumferentialsurface of the electroforming portion.
 3. A fluid dynamic bearing deviceaccording to claim 1, wherein the dynamic pressure generating portionincludes a plurality of dynamic pressure grooves.
 4. A fluid dynamicbearing device according to claim 1, wherein the dynamic pressuregenerating portion includes a plurality of circular arc surfaces.
 5. Afluid dynamic bearing device according to claim 1, wherein theelectroforming portion is provided with a flange.
 6. A fluid dynamicbearing device according to claim 5, wherein an outer circumferentialsurface of the flange has a non-perfect circular shape.
 7. A fluiddynamic bearing device according to claim 5, wherein the flange isformed through plastic deformation of the electroforming portion.
 8. Afluid dynamic bearing device according to claim 5, wherein the flange isformed on at least one of both axial ends of a bearing surface.
 9. Afluid dynamic bearing device according to claim 8, wherein theelectroforming portion has an axial length L2 before the plasticdeformation of the flange and an axial length L1 after the plasticdeformation of the flange, which satisfy a following relationshipexpressed as 0<A/L1≦0.5 (where A=L2−L1).
 10. A fluid dynamic bearingdevice according to claim 1, wherein: the bearing member is formed of aresin used as an injection molding material; and a mold shrinkage factorof the resin is set within a range of 0.02% to 2.0% inclusive.
 11. Afluid dynamic bearing device according to claim 1, wherein the bearingmember is formed of metal used as an injection molding material.
 12. Afluid dynamic bearing device according to claim 1, wherein theelectroforming portion is provided with a thrust bearing surface formedthereon for supporting an end portion of the shaft member in a thrustdirection.
 13. A fluid dynamic bearing device according to claim 12,wherein the thrust bearing surface comes into contact with and supportsthe shaft member in the thrust direction.
 14. A fluid dynamic bearingdevice according to claim 12, wherein any one of the thrust bearingsurface and an end surface of the shaft member opposed to the thrustbearing surface is provided with a plurality of dynamic pressure groovesformed thereon.
 15. A fluid dynamic bearing device according to claim 1,wherein, the dynamic pressure generating portion has a cross section ina radius direction, in which a radius r1 of a virtual circle inscribedto the inner circumferential surface of the electroforming portion islarger than a radius r2 of a virtual circle circumscribed to the outercircumferential surface of the shaft member, and in which a sum of acircularity of the inner circumferential surface of the electroformingportion and a circularity of the outer circumferential surface of theshaft member is 4 μm or less.
 16. A fluid dynamic bearing deviceaccording to claim 1, wherein the shaft member is a master shaft used ata time of forming the electroforming portion.
 17. A fluid dynamicbearing device according to claim 1, wherein the shaft member is amember different from a master shaft used at a time of forming theelectroforming portion.
 18. A fluid dynamic bearing device according toclaim 16, comprising: a thrust bearing surface for supporting an endportion of the shaft member in a thrust direction; a forming portion inwhich the thrust bearing surface is formed, on one end of the mastershaft; and a thrust bearing surface on another end of the master shaft.19. A motor, comprising the fluid dynamic bearing device according toclaim
 1. 20. A method of manufacturing a bearing member, comprising thesteps of: fabricating a master shaft having a forming portioncorresponding to a shape of a dynamic pressure generating portion on anouter circumference of the master shaft; forming an electroformingportion on the outer circumference of the master shaft including theforming portion; performing injection molding while inserting theelectroforming portion after the electroforming portion is formed; andseparating the master shaft and the electroforming portion from eachother after the injection molding.
 21. A method of manufacturing thebearing member according to claim 20, wherein the master shaft and theelectroforming portion are separated from each other by giving themaster shaft and the electroforming portion a difference in thermalexpansion amount.
 22. A method of manufacturing the bearing memberaccording to claim 20, wherein, when the injection molding is performedwhile inserting the electroforming portion, the electroforming portionis plastically deformed by clamping a metal mold to form a flange on theelectroforming portion.